1. Field of the Invention
The present invention relates to a cathode, for a lithium ion secondary battery, formed of a graphite/carbon composite including powdery crystal graphite orientedly bound with a binder which is a low crystallinity carbonaceous material, and a method for manufacturing such a cathode. Also it relates to a lithium ion battery provided with the cathode.
2. Description of the Related Art
All batteries wherein metallic lithium is used as a material for activating a cathode are called lithium batteries. Correctly, a material for activating an anode is combined therewith to form a "manganese dioxide-lithium battery".
The lithium battery which has been recently put into practice, generates a high voltage about twice that of the conventional dry cell, has a large capacity and has a long shelf life of five years or more, whereby it has become widely used even though it is expensive. As a result of the recent development of the miniaturization technology for electronic apparatuses, there is an increased need for the miniaturization of batteries used for a power source thereof, which stimulates the research and development for novel lithium ion secondary batteries having a higher energy density, a larger capacity and a higher electro-motive force.
There are problems to be solved, however, in the conventional lithium batteries since metallic lithium in, for example, a foil form is used as a material for activating the cathode. That is, metallic lithium is eluted in an electrolytic solution in accordance with the discharge and is deposited again during the charge. At that time, lithium is deposited to form dendrites or particulates. Dendrites may cause a short-circuit accident or a reduction in capacity when it falls off, which results in the deterioration of the cycling characteristic and a reduction in safety. Since dendrites are liable to be generated when a large current flows, a boosting charge can have an adverse effect on the cycling life of the battery.
To solve such a drawback, it is proposed to use materials capable of occluding lithium such as lithium/aluminum alloy or Wood's metal, for forming a cathode. However, these materials have a poor processibility as electrode materials.
Of the materials capable of occluding lithium, carbon is practically the most suitable for a cathode, and studies have recently been conducted for causing lithium to be carried on various carbonaceous materials such as graphite or the like. Japanese Unexamined Patent Publication (Kokai) No. 57-208079 discloses a anode in which a paste including powdery graphite mixed with a binder is coated on a metal foil collector. This is based on a fact in that, if a battery is charged while using graphite as a cathode, lithium in an anode is electro-chemically intercalated between the graphite layers in the cathode, which lithium can be deintercalated therefrom into an electrolytic solution as ions during the discharge and restored to the anode.
The theoretical value of the discharging capacity of metallic lithium is 3860 mAh/g. However, the theoretical value of the discharging capacity of graphite is 372 mAh/g, even though it could occlude lithium-containing substance even including LiC.sub.6, which value is less than 1/10 of that of metallic lithium. Accordingly, a higher capacity cannot always be expected even if the cathode proposed in the prior art, in which lithium is carried by graphite, is used. Also, there is a problem that, when the cathode is formed solely of graphite, the lithium-occlusion capacity becomes low to cause the charge/discharge capacity to be too small to allow use as a lithium ion battery.
If the cathode is formed solely of graphite, it has been found that the decomposition of propylene carbonate (PC) used as an electrolyte proceeds at a Coulomb efficiency of 100%, resulting in the problem of lithium-occlusion.
Also, in the cathode formed solely of graphite, graphite crystals repeatedly expand and contract in the direction of the C axis during the intercalation/deintercalation of lithium relative to the graphite layers due to the charge and discharge. If the charging/discharging cycle is repeated many times, the crystal structure of graphite is maintained in the expanded state, which results in the lowering of the adhesion between a collector and the powdery graphite in the cathode to cause a fall-off of powdery graphite from the collector, which in turn deteriorates the collector efficiency of the cathode or the charging/discharging property of the battery.
Other than the cathode formed solely of graphite, Japanese Unexamined Patent Publication (Kokai) No. 62-122066 discloses a anode formed of a carbonaceous material obtained by the carbonization of organic substance. Such an electrode, however, is defective in that it has a small lithium-occlusion capacity which is far lower than the theoretical value, if it is formed of prior art carbonaceous material.